This invention relates to a method for inhibiting the corrosion of metal surfaces, preferably iron and iron alloys and more preferably low carbon steel, in contact with an aqueous system, preferably a cooling water or related water-handling system, and preventing the precipitation in such systems of scale-forming calcium salts, particularly calcium phosphate.
Aqueous systems, such as cooling water and related water-handling systems, include cooling towers and associated pumps, heat exchangers, and pipelines, heating systems, gas scrubbing systems and other similar systems. Problems commonly encountered in these water handling systems include not only the electrochemical corrosion of iron and iron alloys in contact with the circulating water but also the precipitation of potentially scale-forming calcium salts. These two processes are, in fact, very closely related because methods of chemical treatment intended to control one of these problems often aggravate the difficulty caused by the other.
However, we have discovered a method and composition for treating the water in such systems that can both inhibit corrosion and prevent the precipitation of scale-forming calcium salts. It is believed that our method and composition can be effective over a wider range of water conditions than are the methods of the prior art, and the objectives of the invention can be achieved without the negative impact on the environment of some of the prior methods.
For many years, the most common method of controlling corrosion in cooling water and related water-handling systems was to treat the water with hexavalent chromium salts, such as sodium chromate. At the same time, scaling due to slightly soluble calcium salts was prevented by treating the water with mineral acids, such as sulfuric acid, to keep the pH low enough to prevent the precipitation of the scale-forming calcium salts. Improvements in this technology over the years included the use of zinc salts and phosphates in combination with the chromates, which could provide good corrosion control at reduced chromate concentrations. However, because of environmental concerns over the discharge of even small amounts of hexavalent chromium in cooling water effluents, new methods continued to be sought that would provide total corrosion inhibition without the use of hexavalent chromium.
Some of the ways that this has been achieved include the use of various combinations of zinc salts, phosphates, polyphosphates, and organic phosphonic acid derivatives and their salts. However, all of these methods in the prior art have certain disadvantages, such as requiring close control of the pH to keep it within a very narrow range or using special additives or dispersants to prevent the precipitation of scale-forming salts like calcium phosphate.
One commonly used method of cooling water treatment is described by Geiger and May in U.S. Pat. No. 4,305,568. In the Geiger and May patent, it is said at col. 10, beginning at line 9, that the aqueous medium must have a pH of 5.5 and above and must also contain calcium ion concentrations, preferably about 15 parts per million. The compositions disclosed in the Geiger and May patent contain a water soluble orthophosphate, a water-soluble polymer composed primarily of moieties derived from acrylic acid and moieties derived from an hydroxy lower alkyl acrylate. Water-soluble organo-phosphonic acid derivatives may also be included.
In the Geiger and May method, as practiced, relatively high concentrations of the orthophosphate are maintained in the circulating water to inhibit corrosion by passivating ferrous alloy surfaces in contact with the water, and the formation of calcium phosphate scale is inhibited by the simultaneous use of the copolymers of acrylic acid and hydroxyalkyl acrylates. The latter copolymers were reported by Godlewski in U.S. Pat. No. 4,029,577 to be effective antiprecipitants for calcium phosphate. As we understand it, the main disadvantages of the method of Geiger and May in practice are that it requires the use of relatively high concentrations of orthophosphate and careful control of the calcium concentration in the system. Such requirements complicate trouble-free control of the system.
Another type of method of corrosion control that avoids the problem of calcium phosphate deposition involves the use of what is known in the art as "all organic" corrosion inhibitors. Technology of this type is described in U.S. Pat. Nos. 4,317,744 and 4,406,811. In this type of method, corrosion control is obtained by using organic phosphonic acids (or blends of phosphonic acids) along with aromatic azoles and various water-soluble polymers (polyacrylic acid, polymethacrylic acid). No calcium phosphate precipitation can occur because no orthophosphate is present in the treatment. However, as we understand it, in practice, methods of this type give limited corrosion protection and require relatively large amounts of inhibitor, and thus have limited economic feasibility.
Despite the limitations of the existing "all organic" inhibitors, organic phosphonic acids and phosphonates have been found effective as corrosion inhibitor components when used in combination with other substances. The evolution of phosphonic acid corrosion control technology began with the direct application of phosphonic acids or their water-soluble salts as corrosion inhibitors. The disadvantages of this technology were twofold. First, the phosphonic acids or phosphonates commonly available required very high feedrates to obtain acceptable corrosion inhibition. In addition, localized corrosion, known in the art as "pitting", could result from insufficient passivation due to poor film formation when the phosphonic acids were used alone.
This problem was recognized by Carter (U.S. Pat. No. 3,837,803) who overcame it by utilizing orthophosphate with a water-soluble organo-phosphonic acid compound selected from a wide range of phosphonic acids. However, Carter acknowledged two significant restrictions in the use of his method. First, as explained at column 3, beginning at line 4, the cooling water has to contain at least 50 ppm of calcium ion to allow the use of this method without the supplemental addition of a metal cation of the group zinc, nickel, cobalt, cadmium or chromium. The undesirable environmental impact of these heavy metal cations is significant, and, as Carter pointed out, governmental regulations often require that they be avoided in effluents. In the teachings of Carter at column 3, one way of getting around the use of these metal cations, at least in systems containing more than 4 ppm of calcium, is to maintain system pH in the range of 8.5 to 9.0. However, as pointed out by Carter at column 3, lines 34-37, this high pH can lead to the delignification of cooling tower wood and may require the supplemental feed of alkali, both of which are undesirable.
A second major drawback of the method of Carter is his statement at col. 3, lines 60-65, that it is desirable to use very tight pH controls when calcium concentrations exceed 80 ppm. Specifically, Carter suggests a pH between 7.1 and 7.5 when calcium ion exceeds this level. This is a very significant disadvantage in almost all systems, because calcium levels often vary over broader ranges than 50 to 80 ppm. Carter requires one narrow pH range for calcium concentrations below 50 ppm and suggests a different but still narrow pH range for calcium concentrations above 80 ppm. Also, when calcium is above 80 ppm, the suggested restriction of pH to the 7.1-7.5 range is difficult to maintain on a consistent basis in most practical operating cooling water systems.
Other combinations of phosphonic acids and inorganic compounds are also mentioned in the prior art. Polyphosphates and polyacrylic acids were combined with phosphonic acids by Gaupp (U.S. Pat. No. 3,992,318). Polyphosphates and polymaleic anhydride or polymaleic acid were combined with phosphonic acids by Sexsmith (U.S. pat. No. 4,105,581). However, because polyphosphates revert in time to orthophosphates, it is believed that, in practice, these methods also cause problems with calcium phosphate precipitation.
We have discovered a new way of overcoming the problems of, and obviating the disadvantages of the prior art. We have found that use of a certain combination of phosphonic acids (or their water-soluble salts) along with orthophosphate, preferably in low concentrations, will provide good corrosion inhibition and simultaneously inhibit the precipitation of calcium phosphate. The present invention, moreover, is effective in both high and low pH waters and in waters with either low or high concentrations of calcium.